More than seventy percent of Americans over the age of sixty are classified as overweight or obese, and the future incidence of these conditions is expected to rise. Although it is unclear why older adults are predisposed to weight gain, decreased total energy expenditure may contribute to positive energy balance. It is also possible that age-related impairments in energy intake regulation result in the inability to appropriately adjust food intake to meet energy requirements with advancing age. The purpose of these investigations was to determine the influence of age and habitual physical activity on acute regulation of energy intake. Secondary objectives were to determine if there are sex differences in energy intake regulation, and to determine if pre-meal water consumption decreases meal energy intake in young and older adults. To achieve these objectives, the ability to spontaneously adjust energy intake at a meal under "preloading" conditions in which a yogurt shake or water was consumed prior to the meal was determined. We hypothesized that older adults would demonstrate less accurate energy intake regulation than younger adults, but that energy intake dysregulation would be attenuated in physically active older adults. We also expected that young men would have higher accuracy of energy intake regulation compared to young women matched for dietary cognitive restraint and cardiorespiratory fitness, and that pre-meal water consumption would decrease meal energy intake in young and older adults. Our main finding was that energy intake regulation is significantly impaired in older compared to younger adults, and that habitual physical activity improves short-term, but not acute, energy intake regulation. We also found that young men demonstrate significantly higher accuracy of energy intake regulation compared to young women. Lastly, we determined that pre-meal water consumption significantly decreases meal energy intake in older, but not young, adults. Overall, these results indicate that acute energy intake regulation is less accurate with advancing age, but that regular physical activity improves short-term energy intake regulation. Additionally, sex appears to influence energy intake regulation, and water consumption is a potential strategy to reduce energy intake in older adults.